Organic light emitting diode display with surface plasmon outcoupling

ABSTRACT

An active matrix organic light emitting diode (OLED) display includes a substrate; a thin-film transistor (TFT) layer formed on the substrate; a layer defining a periodic grating structure; a first electrode layer formed over the grating structure and conforming to the grating structure; an OLED material layer formed over the first electrode layer and conforming to the grating structure; and a second electrode layer formed over the OLED material layer and conforming to the grating structure, wherein the first and/or second electrode layers are metallic layers, whereby the periodic grating structure induces surface plasmon cross coupling in the metallic electrode layer(s).

FIELD OF THE INVENTION

The present invention relates to organic light emitting diode displays,and more particularly to increasing the light output from the emissivelayers.

BACKGROUND OF THE INVENTION

Organic light emitting diodes (OLED are a promising technology forflat-panel displays. The technology relies upon thin-film layers ofmaterials coated upon a substrate. However, as is well known, much ofthe light output from the emissive elements in the OLED is absorbedwithin the device. Because the light emission from the OLED isLambertian, light is emitted equally in all directions so that some ofthe light is emitted forward to a viewer, some is emitted to the back ofthe device and is either reflected forward to a viewer or absorbed, andsome of the light is emitted laterally and trapped and absorbed by thevarious layers comprising the device. In general, up to 80% of the lightmay be lost.

A variety of techniques have been proposed to improve the out-couplingof light from thin-film displays. For example, diffraction gratings havebeen proposed to control the attributes of light emission from thinpolymer films by inducing Bragg scattering of light that is guidedlaterally through the emissive layers, see Modification of polymer lightemission by lateral microstructure” by Safonov et al., Synthetic Metals116, 2001, pp. 145-148, and “Bragg scattering from periodicallymicrostructured light emitting diodes” by Lupton et al., Applied PhysicsLetters, Vol. 77, No. 21, Nov. 20, 2000, pp. 3340-3342. Brightnessenhancement films having diffractive properties and surface and volumediffusers are described in WO0237568 A1 entitled “Brightness andContrast Enhancement of Direct View Emissive Displays” by Chou et al.Mar. 2, 2001.

The use of micro-cavities and scattering techniques is also known, forexample see “Sharply directed emission in organic electroluminescentdiodes with an optical-microcavity structure” by Tsutsui et al., AppliedPhysics Letters 65, No. 15, Oct. 10, 1994, pp. 1868-1870. However, noneof these approaches capture all, or nearly all, of the light produced.

It has been proposed to use a periodic, corrugated, grating structure toinduce surface plasmon coupling for the light emitting layer in anorganic luminescent device, thereby inhibiting lateral transmission andwave guiding of emitted light while increasing the efficiency and thelight output of the structure. It is theoretically possible to couple upto 93% of the light emitted by the organic luminescent materials in anorganic luminescent device. See “Extraordinary transmission of organicphotoluminescence through an otherwise opaque metal layer via surfaceplasmon cross coupling” by Gifford et al., Applied Physics Letters, Vol.80, No. 20, May 20, 2002. Gifford et al. disclose creating the gratinggeometry for photoluminescent surface plasmon coupling by exposing aphotoresist on glass with an interferometric pattern, followed bydepositing subsequent layers that replicate the underlying surfaceprofile. This approach is not compatible with the current manufacturingtechniques used to make active matrix OLED displays, since fortop-emitting OLED display devices, a layer of thin-film transistors areformed on the substrate prior to forming the OLEDs. For bottom-emittingOLED displays, manufacturing starts with a glass substrate that iscoated with a layer of conductive indium tin oxide (ITO) that ispatterned to provide conductors for thin-film transistors located on thesubstrate. The use of photoresist to create plasmon inducing gratings isproblematical because the photoresist is an electrical insulator,thereby isolating the underlying ITO conductors from the OLED materials.Gifford et al. also suggest that the use of surface plasmon coupling canbe an efficient means for outcoupling electroluminescence in an OLEDdevice by using shadow masks on any desirable substrate. The use ofshadow masks is not practical to create the gratings because of thesmall dimensions of the gratings. They also disclose that they havefabricated an OLED that uses surface plasmon coupling on a siliconsubstrate, but silicon substrates are not conventional or practical forOLED display devices.

There is a need therefore for an improved organic light emitting diodedisplay structure that avoids the problems noted above and improves theefficiency of the display for practical devices.

SUMMARY OF THE INVENTION

The need is met according to the present invention by providing a activematrix organic light emitting diode (OLED) display that includes asubstrate; a thin-film transistor (TFT) layer formed on the substrate; alayer defining a periodic grating structure; a first electrode layerformed over the grating structure and conforming to the gratingstructure; an OLED material layer formed over the first electrode layerand conforming to the grating structure; and a second electrode layerformed over the OLED material layer and conforming to the gratingstructure, wherein the first and/or second electrode layers are metalliclayers, whereby the periodic grating structure induces surface plasmoncross coupling in the metallic electrode layer(s).

In one embodiment, the OLED display is a top-emitting active matrixorganic light emitting diode (OLED) display that includes a substrate; athin-film transistor (TFT) layer formed on the substrate; an insulatinglayer formed over the TFT layer, the insulating layer defining aperiodic grating structure, a first electrode layer formed over theinsulating layer and conforming to the grating structure; an OLEDmaterial layer formed over the first electrode layer and conforming tothe grating structure; and a second electrode layer formed over the OLEDmaterial layer and conforming to the grating structure, wherein thefirst and/or second electrode layers are metallic layers, whereby theperiodic grating structure induces surface plasmon cross coupling in themetallic electrode layer(s).

In an alternative embodiment, the OLED display is a bottom-emittingactive matrix organic light emitting diode (OLED) display that includesa substrate; a first electrode layer formed on the substrate, the firstelectrode layer having first portions defining a periodic gratingstructure and second portions free of such a grating structure; athin-film transistor (TFT) layer formed on the second portions of thefirst electrode layer; an OLED material layer formed over the firstportions of the first electrode layer and conforming to the gratingstructure; and a second electrode layer formed over the OLED materiallayer and conforming to the grating structure, wherein the first and/orsecond electrode layers are metallic layers, whereby the periodicgrating structure induces surface plasmon cross coupling in the metalliclayer(s).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional diagram of a top-emitting OLEDdisplay according to the present invention;

FIG. 2 is a schematic cross sectional diagram of a prior arttop-emitting OLED display;

FIG. 3 is a schematic cross sectional diagram of a prior artbottom-emitting OLED display;

FIG. 4 is a schematic cross sectional diagram of a bottom-emitting OLEDdisplay according to the present invention; and

FIG. 5 is a schematic cross sectional diagram of a generic prior OLEDstructure.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 2, a prior art top-emitting OLED display device 10includes a substrate 12, and a thin-film transistor (TFT) active matrixlayer 14 comprising an array of TFTs that provides power to OLEDelements. A patterned first insulating layer 16 is provided over the TFTactive matrix layer, and an array of first electrodes 18 are providedover a planarized insulating layer 16 and in electrical contact with theTFT active matrix layer. A patterned second insulating layer 17 isprovided over the array of first electrodes 18 such that at least aportion of the each of the first electrodes 18 is exposed.

Over the first electrodes and insulating layers are provided red, green,and blue-emitting organic OLED elements, 19R, 19G, and 19B,respectively. These elements are composed of further layers as describedin more detail below. Herein, the collection of OLED elements, includinghole injection, hole transport, and electron transport layers may alsobe referred to as the OLED layer 19. The light-emitting area isgenerally defined by the area of the first electrode 18 in contact withthe OLED elements. Over the OLED layer 19 is provided a transparent,common second electrode 30 that has sufficient optical transparency toallow transmission of the generated red, green, and blue light. Anoptional second electrode protection layer 32 may be provided to protectthe electrode and underlying layers. Each first electrode in combinationwith its associated OLED element and second electrode is herein referredto as an OLED. A typical top-emitting OLED display device comprises anarray of OLEDs wherein each OLED emits red, green or blue. A gap,generally filled with inert gas or a transmissive polymer material,separates the electrode protection layer from an encapsulating cover 36.The encapsulating cover 36 may also be a layer deposited directly on thecommon second electrode 30 or the optional second electrode protectionlayer 32.

In operation, the thin-film transistors in TFT layer 14 control thecurrent between the first electrodes 18, each of which can beselectively addressed, and the common second electrode 30. Holes andelectrons recombine within the OLED elements to emit light 24 R, G and Bfrom the light emitting elements 19 R, G and B respectively. Because thelayers are so thin, typically several hundred angstroms, they arelargely transparent.

Referring to FIG. 1 a top-emitter embodiment of the present inventionincludes a substrate 12, TFT layer 14, an insulating layer 16, firstpatterned electrode 18, and second insulating layer 17. ConventionalOLED layers 19 are deposited upon the insulating layer 17 and firstpatterned metal electrodes 18. A second, common electrode 30 andprotection layer 32 are deposited above the OLED layers 19. The display10 is encapsulated with an encapsulating cover or layer 36.

The insulating layer 16 is made of conventional materials but is not aconventional planarization layer as in the prior art but rather has aperiodic physical grating structure that makes the layer thicker in somelocations and thinner in others. The size and period of the gratingstructure is selected to be effective to cause surface plasmon crosscoupling in overlying metallic layers that conform to the gratingstructure. In particular, the first patterned metal electrode 18 has asimilar periodic structure, as do the OLED layers 19. The secondelectrode layer 30 is likewise conformable to the grating structure, butthe top surface of the second electrode layer 30 or layers above thesecond electrode 30 may, or may not, conform to the periodic gratingstructure.

In a preferred embodiment, the periodic grating structure of theinsulating layer 16 differs for each of the red, green, and blue OLEDlight emitting areas 19R, 19G, and 19B respectively. The period of thegrating structure is dependent on the frequency of light emitted by theOLED materials. For example, the periodic structure of the insulatinglayer 16 can have a period ranging from 200 to 1000 nm. The height ofthe physical structure is about 100 nm although larger or smallerheights are possible; the minimum thickness of the insulating layer mustbe sufficient to provide good insulation between the first patternedmetal electrode 18 and the thin-film electronics devices 14. The periodand heights of the periodic grating structure affect the frequency ofoptimum cross-coupling and angular dependence. In general, the OLEDelement layer should be as thin as possible to cause as much energy aspossible to undergo surface plasmon cross coupling in the metalliclayers. The insulating layer 16 may be reflective or transmissive, ormay be opaque to increase the contrast of the device. The insulatinglayer 16 is made by conventional means, for example photo-lithography.

In operation, current is passed via the electrodes 18 and 30 through thelight emitting elements 19 causing light to be emitted both upwardthrough second electrode 30 and downward toward the substrate. Theperiodic structure of the first patterned metal electrode 18 and theOLED layer 19 causes surface plasmon cross coupling between the layers.The surface plasmon effect has the additional benefit of reducing theabsorption of light in the electrode, further increasing the lightoutput from the device. The emission from the OLED device is no longerLambertian, but is highly directional along an axis perpendicular to thedisplay. The light emitted forward is seen by a viewer. The lightemitted backward is either absorbed or reflected by the insulatinglayer.

The present invention may be applied to both a top-emitter (whereinlight is emitted through the cover as described above) or abottom-emitter (wherein light is emitted through the substrate). In thebottom emitter case, the periodic grating structure may be createddirectly upon the substrate 12, or to insulating or conducting layersapplied to the substrate. Referring to FIG. 3, a prior artbottom-emitter device uses a patterned conductive layer 13 of indium tinoxide (ITO) deposited on the substrate to conduct current to the lightemitting areas.

Referring to FIG. 4, in a bottom-emitter OLED display according to thepresent invention, the ITO is provided with a periodic grating patternsimilar to that of the insulating layer 16 of the top emitter in theareas where light is emitted. The grating pattern is created in the ITOlayer using well-known photolithography techniques. A thin metalelectrode layer 15 is deposited on the corrugated ITO, the organicmaterials are conformably deposited on the metal layer, and theremainder of the depositions are as described earlier. The thin metalelectrode 15 may be omitted, but surface plasmon coupling will not besupported in the ITO layer alone.

Because the emitted light 24 is polarized and has an angular dependenceon frequency, a diffuser may be included in the display 10 to mitigatethe effect of color aberrations. This diffuser may be applied to theexterior of the device, for example, or the diffuser may be incorporatedinto the cover (for a top emitter) or the substrate (for a bottomemitter).

In another embodiment of the present invention, the period of thestructure of the insulating layer 16 and conformable layers depositedupon it may be constant across the device rather than different for eachindividual color 19R, G, and B. This simplifies the construction of thedevice with some loss in efficiency of the light output and angulardependence of frequency.

Details of the OLED materials, layers, and architecture are described inmore detail below.

GENERAL DEVICE ARCHITECTURE

The present invention can be employed in most OLED deviceconfigurations. These include very simple structures comprising a singleanode and cathode to more complex devices, such as passive matrixdisplays comprised of orthogonal arrays of anodes and cathodes to formpixels, and active-matrix displays where each pixel is controlledindependently, for example, with thin-film transistors (TFTs).

There are numerous configurations of the organic layers wherein thepresent invention can be successfully practiced. A typical structure isshown 10 in FIG. 5 and is comprised of a substrate 12, an anode 103, ahole-injecting layer 105, a hole-transporting layer 107, alight-emitting layer 109, an electron-transporting layer 111, and acathode 113. These layers are described in detail below. Note that thesubstrate may alternatively be located adjacent to the cathode, or thesubstrate may actually constitute the anode or cathode. The organiclayers between the anode and cathode are conveniently referred to as theorganic EL element. The total combined thickness of the organic layersis preferably less than 500 nm.

The anode and cathode of the OLED are connected to a voltage/currentsource 250 through electrical conductors 260. The OLED is operated byapplying a potential between the anode and cathode such that the anodeis at a more positive potential than the cathode. Holes are injectedinto the organic EL element from the anode and electrons are injectedinto the organic EL element at the anode. Enhanced device stability cansometimes be achieved when the OLED is operated in an AC mode where, forsome time period in the cycle, the potential bias is reversed and nocurrent flows. An example of an AC driven OLED is described in U.S. Pat.No. 5,552,678.

Substrate

The OLED device of this invention is typically provided over asupporting substrate where either the cathode or anode can be in contactwith the substrate. The electrode in contact with the substrate isconveniently referred to as the bottom electrode. Conventionally, thebottom electrode is the anode, but this invention is not limited to thatconfiguration. The substrate can either be light transmissive or opaque,depending on the intended direction of light emission. The lighttransmissive property is desirable for viewing the EL emission throughthe substrate. Transparent glass or plastic is commonly employed in suchcases. For applications where the EL emission is viewed through the topelectrode, the transmissive characteristic of the bottom support isimmaterial, and therefore can be light transmissive, light absorbing orlight reflective. Substrates for use in this case include, but are notlimited to, glass, plastic, semiconductor materials, silicon, ceramics,and circuit board materials. Of course it is necessary to provide inthese device configurations a light-transparent top electrode.

Anode

When EL emission is viewed through anode 103, the anode should betransparent or substantially transparent to the emission of interest.Common transparent anode materials used in this invention are indium-tinoxide (ITO), indium-zinc oxide (IZO) and tin oxide, but other metaloxides can work including, but not limited to, aluminum- or indium-dopedzinc oxide, magnesium-indium oxide, and nickel-tungsten oxide. Inaddition to these oxides, metal nitrides, such as gallium nitride, andmetal selenides, such as zinc selenide, and metal sulfides, such as zincsulfide, can be used as the anode. For applications where EL emission isviewed only through the cathode electrode, the transmissivecharacteristics of anode are immaterial and any conductive material canbe used, transparent, opaque or reflective. Example conductors for thisapplication include, but are not limited to, gold, iridium, molybdenum,palladium, and platinum. Typical anode materials, transmissive orotherwise, have a work function of 4.1 eV or greater. Desired anodematerials are commonly deposited by any suitable means such asevaporation, sputtering, chemical vapor deposition, or electrochemicalmeans. Anodes can be patterned using well-known photolithographicprocesses. Optionally, anodes may be polished prior to application ofother layers to reduce surface roughness so as to minimize shorts orenhance reflectivity.

Hole-Injecting Layer (HIL)

While not always necessary, it is often useful to provide ahole-injecting layer 105 between anode 103 and hole-transporting layer107. The hole-injecting material can serve to improve the film formationproperty of subsequent organic layers and to facilitate injection ofholes into the hole-transporting layer. Suitable materials for use inthe hole-injecting layer include, but are not limited to, porphyriniccompounds as described in U.S. Pat. No. 4,720,432, plasma-depositedfluorocarbon polymers as described in U.S. Pat. No. 6,208,075, and somearomatic amines, for example, m-MTDATA(4,4′,4″-tris[(3-methylphenyl)phenylamino]triphenylamine). Alternativehole-injecting materials reportedly useful in organic EL devices aredescribed in EP 0 891 121 A1 and EP 1 029 909 A1.

Hole-Transporting Layer (HTL)

The hole-transporting layer 107 contains at least one hole-transportingcompound such as an aromatic tertiary amine, where the latter isunderstood to be a compound containing at least one trivalent nitrogenatom that is bonded only to carbon atoms, at least one of which is amember of an aromatic ring. In one form the aromatic tertiary amine canbe an arylamine, such as a monoarylamine, diarylamine, triarylamine, ora polymeric arylamine. Exemplary monomeric triarylamines are illustratedby Klupfel et al. in U.S. Pat. No. 3,180,730. Other suitabletriarylamines substituted with one or more vinyl radicals and/orcomprising at least one active hydrogen containing group are disclosedby Brantley et al. U.S. Pat. Nos. 3,567,450 and 3,658,520.

A more preferred class of aromatic tertiary amines are those whichinclude at least two aromatic tertiary amine moieties as described inU.S. Pat. Nos. 4,720,432 and 5,061,569. The hole-transporting layer canbe formed of a single or a mixture of aromatic tertiary amine compounds.Illustrative of useful aromatic tertiary amines are the following:

1,1-Bis(4-di-p-tolylaminophenyl)cyclohexane

1,1-Bis(4-di-p-tolylaminophenyl)-4-phenylcyclohexane

4,4′-Bis(diphenylamino)quadriphenyl

Bis(4-dimethylamnino-2-methylphenyl)-phenylmethane

N,N,N-Tri(p-tolyl)amine

4-(di-p-tolylamino)-4′-[4-(di-p-tolylamino)-styryl]stilbene

N,N,N′,N′-Tetra-p-tolyl-4-4′-diaminobiphenyl

N,N,N′,N′-Tetraphenyl-4,4′-diaminobiphenyl

N,N,N′,N′-tetra-1-naphthyl-4,4′-diaminobiphenyl

N,N,N′,N′-tetra-2-naphthyl-4,4′-diaminobiphenyl

N-Phenylcarbazole

4,4′-Bis[N-(1-naphthyl)-N-phenylamino]biphenyl

4,4′-Bis[N-(1-naphthyl)-N-(2-naphthyl)amino]biphenyl

4,4″-Bis[N-(1-naphthyl)-N-phenylamino]p-terphenyl

4,4′-Bis[N-(2-naphthyl)-N-phenylamino]biphenyl

4,4′-Bis[N-(3-acenaphthenyl)-N-phenylamino]biphenyl

1,5-Bis[N-(1-naphthyl)-N-phenylamino]naphthalene

4,4′-Bis[N-(9-anthryl)-N-phenylamino]biphenyl

4,4″-Bis[N-(1-anthryl)-N-phenylamino]-p-terphenyl

4,4′-Bis[N-(2-phenanthryl)-N-phenylamino]biphenyl

4,4′-Bis[N-(8-fluoranthenyl)-N-phenylamino]biphenyl

4,4′-Bis[N-(2-pyrenyl)-N-phenylamino]biphenyl

4,4′-Bis[N-(2-naphthacenyl)-N-phenylamino]biphenyl

4,4′-Bis[N-(2-perylenyl)-N-phenylamino]biphenyl

4,4′-Bis[N-(1-coronenyl)-N-phenylamino]biphenyl

2,6-Bis(di-p-tolylamino)naphthalene

2,6-Bis[di-(1-naphthyl)amino]naphthalene

2,6-Bis[N-(1-naphthyl)-N-(2-naphthyl)amino]naphthalene

N,N,N′,N′-Tetra(2-naphthyl)-4,4″-diamino-p-terphenyl

4,4′-Bis{N-phenyl-N-[4-(1-naphthyl)-phenyl]amino}biphenyl

4,4′-Bis[N-phenyl-N-2-pyrenyl)amino]biphenyl

2,6-Bis[N,N-di(2-naphthyl)amine]fluorene

1,5-Bis[N-(1-naphthyl)-N-phenylamino]naphthalene

4,4′,4″-tris[(3-methylphenyl)phenylamino]triphenylamine

Another class of useful hole-transporting materials includes polycyclicaromatic compounds as described in EP 1 009 041. Tertiary aromaticamines with more than two amine groups may be used including oligomericmaterials. In addition, polymeric hole-transporting materials can beused such as poly(N-vinylcarbazole) (PVK), polythiophenes, polypyrrole,polyaniline, and copolymers such aspoly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) also calledPEDOT/PSS.

Light-Emitting Layer (LEL)

As more fully described in U.S. Pat. Nos. 4,769,292 and 5,935,721, thelight-emitting layer (LEL) 109 of the organic EL element includes aluminescent or fluorescent material where electroluminescence isproduced as a result of electron-hole pair recombination in this region.The light-emitting layer can be comprised of a single material, but morecommonly consists of a host material doped with a guest compound orcompounds where light emission comes primarily from the dopant and canbe of any color. The host materials in the light-emitting layer can bean electron-transporting material, as defined below, a hole-transportingmaterial, as defined above, or another material or combination ofmaterials that support hole-electron recombination. The dopant isusually chosen from highly fluorescent dyes, but phosphorescentcompounds, e.g., transition metal complexes as described in WO 98/55561,WO 00/18851, WO 00/57676, and WO 00/70655 are also useful. Dopants aretypically coated as 0.01 to 10% by weight into the host material.Polymeric materials such as polyfluorenes and polyvinylarylenes (e.g.,poly(p-phenylenevinylene), PPV) can also be used as the host material.In this case, small molecule dopants can be molecularly dispersed intothe polymeric host, or the dopant could be added by copolymerizing aminor constituent into the host polymer.

An important relationship for choosing a dye as a dopant is a comparisonof the bandgap potential which is defined as the energy differencebetween the highest occupied molecular orbital and the lowest unoccupiedmolecular orbital of the molecule. For efficient energy transfer fromthe host to the dopant molecule, a necessary condition is that the bandgap of the dopant is smaller than that of the host material. Forphosphorescent emitters it is also important that the host tripletenergy level of the host be high enough to enable energy transfer fromhost to dopant.

Host and emitting molecules known to be of use include, but are notlimited to, those disclosed in U.S. Pat. Nos. 4,768,292; 5,141,671;5,150,006; 5,151,629; 5,405,709, 5,484,922; 5,593,788; 5,645,948;5,683,823; 5,755,999; 5,928,802; 5,935,720; 5,935,721; and 6,020,078.

Metal complexes of 8-hydroxyquinoline (oxine) and similar derivativesconstitute one class of useful host compounds capable of supportingelectroluminescence. Illustrative of useful chelated oxinoid compoundsare the following:

CO-1: Aluminum trisoxine[alias, tris(8-quinolinolato)aluminum(III)]

CO-2: Magnesium bisoxine[alias, bis(8-quinolinolato)magnesium(II)]

CO-3: Bis[benzo{f}-8-quinolinolato]zinc (II)

CO-4:Bis(2-methyl-8-quinolinolato)aluminum(III)-μ-oxo-bis(2-methyl-8-quinolinolato)aluminum(III)

CO-5: Indium trisoxine[alias, tris(8-quinolinolato)indium]

CO-6: Aluminum tris(5-methyloxine)[alias, tris(5-methyl-8-quinolinolato)aluminum(III)]

CO-7: Lithium oxine[alias, (8-quinolinolato)lithium(I)]

CO-8: Gallium oxine[alias, tris(8-quinolinolato)gallium(III)]

CO-9: Zirconium oxine[alias, tetra(8-quinolinolato)zirconium(IV)]

Other classes of useful host materials include, but are not limited to:derivatives of anthracene, such as 9,10-di-(2-naphthyl)anthracene andderivatives thereof as described in U.S. Pat. No. 5,935,721,distyrylarylene derivatives as described in U.S. Pat. No. 5,121,029, andbenzazole derivatives, for example,2,2′,2″-(1,3,5-phenylene)tris[1-phenyl-1H-benzimidazole]. Carbazolederivatives are particularly useful hosts for phosphorescent emitters.

Useful flourescent dopants include, but are not limited to, derivativesof anthracene, tetracene, xanthene, perylene, rubrene, coumarin,rhodamine, and quinacridone, dicyanomethylenepyran compounds, thiopyrancompounds, polymethine compounds, pyrilium and thiapyrilium compounds,fluorene derivatives, periflanthene derivatives, indenoperylenederivatives, bis(azinyl)amine boron compounds, bis(azinyl)methanecompounds, and carbostyryl compounds.

Electron-Transporting Layer (ETL)

Preferred thin-film-forming materials for use in forming theelectron-transporting layer 111 of the organic EL elements of thisinvention are metal chelated oxinoid compounds, including chelates ofoxine itself (also commonly referred to as 8-quinolinol or8-hydroxyquinoline). Such compounds help to inject and transportelectrons, exhibit high levels of performance, and are readilyfabricated in the form of thin-films. Exemplary oxinoid compounds werelisted previously.

Other electron-transporting materials include various butadienederivatives as disclosed in U.S. Pat. No. 4,356,429 and variousheterocyclic optical brighteners as described in U.S. Pat. No.4,539,507. Benzazoles and triazines are also usefulelectron-transporting materials.

Cathode

When light emission is viewed solely through the anode, the cathode 113used in this invention can be comprised of nearly any conductivematerial. Desirable materials have good film-forming properties toensure good contact with the underlying organic layer, promote electroninjection at low voltage, and have good stability. Useful cathodematerials often contain a low work function metal (<4.0 eV) or metalalloy. One preferred cathode material is comprised of a Mg:Ag alloywherein the percentage of silver is in the range of 1 to 20%, asdescribed in U.S. Pat. No. 4,885,221. Another suitable class of cathodematerials includes bilayers comprising a thin electron-injection layer(EIL) in contact with the organic layer (e.g., ETL) which is capped witha thicker layer of a conductive metal. Here, the EEL preferably includesa low work function metal or metal salt, and if so, the thicker cappinglayer does not need to have a low work function. One such cathode iscomprised of a thin layer of LiF followed by a thicker layer of Al asdescribed in U.S. Pat. No. 5,677,572. Other useful cathode material setsinclude, but are not limited to, those disclosed in U.S. Pat. Nos.5,059,861, 5,059,862, and 6,140,763.

When light emission is viewed through the cathode, the cathode must betransparent or nearly transparent. For such applications, metals must bethin or one must use transparent conductive oxides, or a combination ofthese materials. Optically transparent cathodes have been described inmore detail in U.S. Pat. No. 4,885,211, U.S. Pat. No. 5,247,190, JP3,234,963, U.S. Pat. No. 5,703,436, U.S. Pat. No. 5,608,287, U.S. Pat.No. 5,837,391, U.S. Pat. No. 5,677,572, U.S. Pat. No. 5,776,622, U.S.Pat. No. 5,776,623, U.S. Pat. No. 5,714,838, U.S. Pat. No. 5,969,474,U.S. Pat. No. 5,739,545, U.S. Pat. No. 5,981,306, U.S. Pat. No.6,137,223, U.S. Pat. No. 6,140,763, U.S. Pat. No. 6,172,459, EP 1 076368, U.S. Pat. No. 6,278,236, and U.S. Pat. No. 6,284,393. Cathodematerials are typically deposited by evaporation, sputtering, orchemical vapor deposition. When needed, patterning can be achievedthrough many well known methods including, but not limited to,through-mask deposition, integral shadow masking, for example, asdescribed in U.S. Pat. No. 5,276,380 and EP 0 732 868, laser ablation,and selective chemical vapor deposition.

Other Common Organic Layers and Device Architecture

In some instances, layers 109 and 111 can optionally be collapsed into asingle layer that serves the function of supporting both light emissionand electron transportation. It also known in the art that emittingdopants may be added to the hole-transporting layer, which may serve asa host. Multiple dopants may be added to one or more layers in order tocreate a white-emitting OLED, for example, by combining blue- andyellow-emitting materials, cyan- and red-emitting materials, or red-,green-, and blue-emitting materials. White-emitting devices aredescribed, for example, in EP 1 187 235, U.S. Pat. No. 20,020,025,419,EP 1 182 244, U.S. Pat. No. 5,683,823, U.S. Pat. No. 5,503,910, U.S.Pat. No. 5,405,709, and U.S. Pat. No. 5,283,182.

Additional layers such as electron or hole-blocking layers as taught inthe art may be employed in devices of this invention. Hole-blockinglayers are commonly used to improve efficiency of phosphorescent emitterdevices, for example, as in U.S. Pat. No. 20,020,015,859.

This invention may be used in so-called stacked device architecture, forexample, as taught in U.S. Pat. No. 5,703,436 and U.S. Pat. No.6,337,492.

Deposition of Organic Layers

The organic materials mentioned above are suitably deposited through avapor-phase method such as sublimation, but can be deposited from afluid, for example, from a solvent with an optional binder to improvefilm formation. If the material is a polymer, solvent deposition isuseful but other methods can be used, such as sputtering or thermaltransfer from a donor sheet. The material to be deposited by sublimationcan be vaporized from a sublimator “boat” often comprised of a tantalummaterial, e.g., as described in U.S. Pat. No. 6,237,529, or can be firstcoated onto a donor sheet and then sublimed in closer proximity to thesubstrate. Layers with a mixture of materials can utilize separatesublimator boats or the materials can be pre-mixed and coated from asingle boat or donor sheet. Patterned deposition can be achieved usingshadow masks, integral shadow masks (U.S. Pat. No. 5,294,870),spatially-defined thermal dye transfer from a donor sheet (U.S. Pat.Nos. 5,688,551, 5,851,709 and 6,066,357) and inkjet method (U.S. Pat.No. 6,066,357).

Encapsulation

Most OLED devices are sensitive to moisture or oxygen, or both, so theyare commonly sealed in an inert atmosphere such as nitrogen or argon,along with a desiccant such as alumina, bauxite, calcium sulfate, clays,silica gel, zeolites, alkaline metal oxides, alkaline earth metaloxides, sulfates, or metal halides and perchlorates. Methods forencapsulation and desiccation include, but are not limited to, thosedescribed in U.S. Pat. No. 6,226,890. In addition, barrier layers suchas SiOx, Teflon, and alternating inorganic/polymeric layers are known inthe art for encapsulation.

Optical Optimization

OLED devices of this invention can employ various well-known opticaleffects in order to enhance its properties if desired. This includesoptimizing layer thicknesses to yield maximum light transmission,providing dielectric mirror structures, replacing reflective electrodeswith light-absorbing electrodes, providing anti glare or anti-reflectioncoatings over the display, providing a polarizing medium over thedisplay, or providing colored, neutral density, or color conversionfilters over the display. Filters, polarizers, and anti-glare oranti-reflection coatings may be specifically provided over the cover oras part of the cover.

The entire contents of the patents and other publications referred to inthis specification are incorporated herein by reference.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

PARTS LIST

10 OLED display device

12 substrate

13 ITO layer

14 TFT layer

15 metal electrode layer

16 insulating layer

17 second insulating layer

18 first electrodes

19 OLED layer

19R red-emitting organic materials layer

19G green-emitting organic materials layer

19B blue-emitting organic materials layer

24 emitted light

24R red light

24G green light

24B blue light

30 second electrode

32 second electrode protection layer

36 encapsulating cover

103 anode

105 hole injection layer

107 hole transport layer

109 light emitting layer

111 electron transport layer

113 cathode

250 voltage/current source

260 electrical conductors

What is claimed is:
 1. A top-emitting active matrix organic lightemitting diode (OLED) display, comprising: a) a substrate; b) athin-film transistor (TFT) layer formed on the substrate; c) aninsulating layer formed over the TFT layer, the insulating layerdefining a periodic grating structure; d) a first electrode layer formedover the insulating layer and conforming to the grating structure; e) anOLED material layer formed over the first electrode layer and conformingto the grating structure; and f) a second electrode layer formed overthe OLED material layer and conforming to the grating structure, whereinthe first and/or second electrode layers are metallic layers, wherebythe periodic grating structure induces surface plasmon cross coupling inthe metallic electrode layer(s).
 2. The display claimed in claim 1,wherein the OLED material layer includes portions for emitting differentcolors of light and the period of the grating structure is different forthe different colors.
 3. The display claimed in claim 1, furthercomprising an encapsulating cover that is light diffusing.
 4. Thedisplay claimed in claim 1, wherein the insulating layer is a lightabsorbing layer.
 5. The display claimed in claim 1, wherein the metalliclayers are opaque.
 6. The display claimed in claim 1, wherein thegrating structure is a two-dimensional grating.
 7. A bottom-emittingactive matrix organic light emitting diode (OLED) display, comprising:a) a substrate; b) a first electrode layer formed on the substrate, thefirst electrode layer having first portions defining a periodic gratingstructure and second portions free of such a grating structure; c) athin-film transistor (TFT) layer formed on the second portions of thefirst electrode layer; d) an OLED material layer formed over the firstportions of the first electrode layer and conforming to the gratingstructure; and e) a second electrode layer formed over the OLED materiallayer and conforming to the grating structure, wherein the first and/orsecond electrode layers are metallic layers, whereby the periodicgrating structure induces surface plasmon cross coupling in the metalliclayer(s).
 8. The display claimed in claim 7, wherein the first electrodelayer is non-metallic and further comprising a metallic layer formed onthe first portions of the first electrode layer and conforming to thegrating structure.
 9. The display claimed in claim 7, wherein the OLEDmaterial layer includes portions for emitting different colors of lightand the period of the grating structure is different for the differentcolors.
 10. The display claimed in claim 7, wherein the substrate islight diffusing.
 11. The display claimed in claim 7, wherein themetallic layers are opaque.
 12. The display claimed in claim 7, whereinthe grating structure is a two-dimensional grating.
 13. The displayclaimed in claim 7, wherein the first electrode layer is indium tinoxide.